Secretory immunoglobulin A (SIgA) is an immune substance which exists in mucosal membranes of the oral cavity, the nostril, the respiratory organs, the gastrointestinal tract and the like, and is the core of mucosal immunity. Although the mucosal membranes are constantly exposed to pathogenic bacteria, allergic substances and the like via ambient air and/or foods, the SIgA plays a critical role in protecting the mucosal membrane therefrom. Specifically, the SIgA protects the mucosal membrane by binding to bacteria and/or viruses and agglomerating the same, as well as binding to toxins, enzymes derived from the bacteria, and the like to detoxify them. The amount of the SIgA secreted is under the control of the brain, the endocrine system and the autonomic nervous system, and is lower in the younger and the elderly, compared to the adults.
On the other hand, natural killer (NK) cells are a type of lymphocyte, which finds and kills cells recognized as nonself, such as cancer cells and virally-infected cells. The number of the NK cells is also small at an early age and in the elderly, which is believed to be a cause of the development of infectious diseases in children and the elderly.
In recent years, development of food products and food product components which enhance the amount of the SIgA secreted or the activity of the NK cells has been sought, and specific strains of lactic acid bacteria and bifidobacteria have been reported to exhibit such functions (see, Patent Documents 1 to 8).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2005-194259
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2007-308419
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-179630
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2008-201708
Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2010-057395
Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2010-130954
Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2010-150206
Patent Document 8: Pamphlet of PCT International Publication No. WO2006-087913